Log cabins can come in any size and some might need to be compliant with building regulations due to their size or other specifications. Investing in a log cabin of this size is a world away from sheds and small summer houses, and some log cabins can be so big that they have their own plumbing facilities as well as other facilities. For this you will have to investigate what planning permission you need before construction begins.
The larger log cabins can cost up to and above £20,000. If you are looking for a more homely alternative to an extension, then this sort of structure could be exactly what you are looking for. With enough space to house double beds and spacious living rooms, it really might be like an entirely new house in the grounds of your own. Structures like this can have multiple uses, from housing guests to creating live-in areas for family or friends. For cabins like this, you should be able to rely on the manufacturer to supply parts that stand up to building regulations.
Insulation for such a structure should the best available, providing the best protection from damp and the elements so electrical equipment is never compromised. Damp proof membranes can stop the onset of any water. Because of the expansive nature of some log cabins, having roof beams that are capable of handling heavy loads of compacted snow is vital.
Log cabins like this are built to be homes, not merely sheds. You should be able to dwell in them all year round and they should look great for decades rather than requiring cosmetic maintenance every couple of years. For an investment like this, it should be treated as another home with all of the relevant security features to protect your investment.